Heterojunction bipolar transistor and method of making heterojunction bipolar transistor

Abstract

A method of making a heterojunction bipolar transistor comprises the steps of: forming a mask layer on a compound semiconductor film by using a photomask for forming an emitter; and forming the emitter by wet-etching the compound semiconductor film by using the mask layer. The photomask has a pattern thereon for forming the emitter. The pattern is defined by a first area R associated with the shape of the emitter to be formed, and a plurality of second areas T 1 to T 4 . Each of the second areas T 1 to T 4 includes first and second sides S 1 and S 2 meeting each other to form an acute angle therebetween, and a third side S 3 in contact with the first area R. In each of the second areas T 1 to T 4 , one side S 3 of the two sides meeting each other to form a right angle therebetween is in contact with one side of the area R, whereas the other side S 1 is connected to another side of the first area R to form a line segment. Using this photomask, an etching mask is arranged such that a side S 11 of the first area R is oriented in [001] direction of the emitter crystal film. When the emitter crystal film is etched by use of the etching mask, a rectangular emitter is obtained.

What is claimed is:  
     
       1. A method of making a heterojunction bipolar transistor, said method comprising the steps of: 
       forming a mask layer on a compound semiconductor film by a photomask for forming an emitter mesa; and  
       forming said emitter mesa by wet-etching said compound semiconductor film by said mask layer;  
       wherein said photomask has a pattern thereon for forming said emitter mesa;  
       wherein said pattern is defined by a first area portion associated with a shape of said emitter mesa to be formed, and a plurality of second area portions; and  
       wherein each of said second area portions has first and second sides meeting each other to form an acute angle therebetween, and a third side in contact with said first area portion.  
     
     
       2. A method of making a heterojunction bipolar transistor according to  claim 1 , wherein said first area portion is defined by a first pair of sides extending in a first direction, and a second pair of sides extending in a second direction intersecting said first direction; and 
       wherein each of said second area portions is arranged such that said third side is in contact with one of said first pair of sides of said first area portion.  
     
     
       3. A method of making a heterojunction bipolar transistor according to  claim 1 , wherein each of said second area portions is formed into a triangle defined by said first to third sides; and 
       wherein said first side of each second area portion is oriented in a direction in which one of said first and second pairs of sides extends.  
     
     
       4. A method of making a heterojunction bipolar transistor according to  claim 1 , wherein said emitter mesa has a first pair of sides extending in a predetermined direction, and a second pair of sides extending in a direction intersecting said predetermined direction. 
     
     
       5. A method of making a heterojunction bipolar transistor according to  claim 4 , wherein an inverted mesa structure is formed at said first pair of sides in said emitter mesa and a normal mesa structure is formed at said second pair of sides in said emitter mesa in said wet-etching step. 
     
     
       6. A method of making a heterojunction bipolar transistor according to  claim 1 , wherein said first area has a pair of edges extending in crystal axis [011] direction of said compound semiconductor film. 
     
     
       7. A method of making a heterojunction bipolar transistor according to  claim 1 , wherein said compound semiconductor film includes an InP semiconductor. 
     
     
       8. A method of making a heterojunction bipolar transistor according to  claim 7 , wherein said mask layer is made of resist. 
     
     
       9. A method of making a heterojunction bipolar transistor according to  claim 7 , further comprising a step of forming an InGaAs base region essentially constituted by eight sides. 
     
     
       10. A method of making a heterojunction bipolar transistor according to  claim 7 , wherein said first area has a pair of edges extending in crystal axis [011] direction of said compound semiconductor film; and 
       said method further comprising the steps of:  
       forming said compound semiconductor film prior to said step of forming said mask layer;  
       forming an InGaAs base region after said step of forming an emitter mesa; and  
       forming an emitter electrode and a pair of base electrodes in a self-alignment fashion with respect to said emitter mesa after said step of forming a said base region.